KAT (kynurenine aminotransferase) II is a primary enzyme in the brain for catalyzing the transamination of kynurenine to KYNA (kynurenic acid). J. Neurochem., 57, 533-540, 1991. KYNA is an effective excitatory amino acid (EAA) receptor antagonist with affinity for the glycine modulatory site of the N-methyl-D-aspartate (NMDA) receptor complex (M. Kessler et al., J. Neurochem., vol. 52, pp. 1319-1328, 1989). As a naturally occurring brain metabolite, KYNA probably serves as a negative endogenous modulator of cerebral glutamatergic function (R. Schwarcz et al., Ann. N.Y. Acad. Sci., vol. 648, pp. 140-153, 1992).
EAA receptors and in particular NMDA receptors are known to play a central role in the function of the mammalian brain (J. C. Watkins and G. L. Collingridge, Eds., The NMDA Receptor, Oxford University Press, Oxford, 1989, p. 242). For example, NMDA receptor activation is essential for cognitive processes, such as, for example, learning and memory (Watkins and Collingridge, supra, pp. 137-151). Therefore, reducing KYNA synthesis by inhibition of its synthetic enzyme may enhance EAA signaling and improve cognitive processes, especially in disease states where NMDA hypofunction is anticipated. Thus, there is a need for compounds which act as KAT II inhibitors to reduce KYNA synthesis within the brain to improve cognitive dysfunction in human disease states.